When using indexed colors, you can only access the first 254 colors of
a 256-color color map. The first two indexes are taken up by the
background and foreground colors.

[Color index 0 = white / color index 1 = black]

[Download NCV_blue_red.rgb
file and put in $NCARG_ROOT/lib/ncarg/colormaps, or use NCARG_COLORMAPS.]

Sample NCL script usages:

;---Using the color map to define colors for contours or vectors
cmap = read_colormap_file("NCV_blue_red") ; 256 x 4 float array
res@cnFillPalette = cmap ; This assures you span all 256 colors
res@vcLevelPalette = cmap
;
; Using this to set the color map will only use the
; first 254 colors of a 256-color map
;
res@cnFillPalette = "NCV_blue_red"
res@vcLevelPalette = "NCV_blue_red"
;---Using the color map to define a color map for the workstation
gsn_define_colormap(wks,"NCV_blue_red")